Resistance of nosocomial and community-acquired pathogens to antimicrobial agents is a serous problem with significant clinical and economic consequences. Many species are resistant to commonly used antimicrobials, and in many cases resistance to multiple classes of drugs is reported. In the past few years, a handful of organisms resistant to all known antimicrobial agents has emerged see, Tenover et al., Am. J. Med. Sci. 311:9-16 (1996)!. Though such organisms are rare, the existence of conditions favoring the development and spread of these organisms forecasts the continued emergence of multi-drug resistance. This problem is further exacerbated by the scarcity of new classes of antimicrobial agents since many pharmaceutical manufacturers have abandoned the discovery of antimicrobial drugs in favor of identifying antifungal and antiviral drugs see, Tenover et al., JAMA 275(4):300-4, 1996!.
Antimicrobial drug resistance has been documented in both gram-negative and gram-positive bacterial pathogens. Among the clinically significant gram-negative bacteria, which account for 60% of infections treated in hospitals, resistance to multiple drugs has been reported in Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Haemophilus influenzae, and Neisseriae gonorrhoeae. Multiple-drug resistance is also found in gram-positive bacteria, such as Staphylococcus aureus, Staphylococcus hemolyticus, and Streptococcus pneumoniae which are isolated from hospital environments. Because resistance to multiple classes of drugs is rapidly spreading among clinically significant bacterial isolates, the clinical and economic consequences of multiple-drug resistance are severe.
A. Antimicrobial Drug Resistance
Three key factors have contributed to the emergence and spread of microbes which are resistant to multiple antimicrobial compounds, including mutations in common resistance genes, exchange of genetic information among microorganisms, and the increased selective pressures in institutional settings and communities.
Mutations in common resistance genes have extended the bacterial spectrum of drug resistance. Resistance genes mostly encode proteins that either inactivate antimicrobial agents or block their site of action. For example, the organism may alter the receptors to which the antimicrobial binds (e.g., conformational changes in penicillin binding proteins (PBPs), such that penicillin won't bind), or it alter the cell membrane such that membrane transport systems are ineffective in transporting the antimicrobial across the cell membrane (e.g., resistance to tetracyclines due to the fact that the drug cannot enter the cell). Alternatively, the organism can develop enzymes which destroy or inactivate the antimicrobial (e.g., .beta.-lactamases which destroy penicillin). In addition, the organism can also alter an enzyme's specificity for its substrate (e.g., sulfonamide-resistant bacteria often have enzymes with a high affinity for para-aminobenzoic acid (PABA), but a low affinity for sulfonamide), or altogether forego its requirement for a particular substrate (e.g., exogenous folic acid may be taken in by sulfonamide-resistant bacteria, thereby by-passing the need to take in PABA as a precursor of folic acid synthesis). Importantly, microorganisms that are resistant to a given drug may also be resistant to other drugs that share certain mechanisms of action. This cross-resistance is usually observed with drugs that are closely related chemically or that have a similar mode of binding or action.
Small changes in resistance determinants (e.g., enzymes) can also have major effects on an organism's resistance profile to drugs which belong to different chemical classes. This is exemplified by the changes in .beta.-lactamases. These enzymes inactivate .beta.-lactam drugs such as penicillin, ampicillin, and cephalothin. Mutant forms of the .beta.-lactamases, which are referred to as extended-spectrum .beta.-lactamases (ESBLs) and which are capable of inactivating the chemically unrelated extended-spectrum cephalosporins and monobactams were reported as early as 1982. Only three amino acid differences, which reflect point mutations in the coding sequence of the .beta.-lactamase, exist between the ESBL and wild-type .beta.-lactmases.
The problem of multiple-resistant bacteria is compounded by the exchange of genetic information between bacteria. Bacteria exchange information by transformation (i.e., the uptake of naked DNA), transduction (i.e., transfer of DNA by bacteriophage), and conjugation (i.e., cell-to-cell contact). The exchange of extrachromosomal elements such as plasmids and transposons during conjugation is the most common method of resistance transfer. Although conjugation was previously thought to be limited to gram-negative bacilli, a similar transfer process has been extensively documented for gram-positive organisms whereby plasmids or independent transposable elements, often carrying multiple-resistance genes, move from one organism to another. The transfer process extends even between gram-negative and gram-positive organisms. For example, Campylobacter coli and enterococci have been shown to exchange aminoglycoside resistance genes {Trieu-Cuot et al., EMBO J. 4:3583-3587 (1985)!. Thus, a susceptible strain can acquire resistance from another resistant species or genus.
Environmental pressures encourage the emergence or acquisition of new mutations. Such pressures include the extended and prophylactic use of antimicrobials in communities, hospitals, nursing homes, day care centers and animal feedlots. In addition, many antimicrobials are bacteriostatic rather than bacteriocidal. Organisms exposed to bacteriostatic drugs remain viable, although their growth is inhibited. Because they remain viable, these organisms are provided with the opportunity to develop mechanisms of resistance to the drug. For example, the use in hospitals of antimicrobials prone to select altered resistance traits results in hospitalized patients, who are usually immunocompromised, quickly becoming colonized with resistant strains.
B. Addressing Antimicrobial Drug Resistance
Attempts to minimize the impact of multiple-drug resistance have focused on barrier isolation, improvement of antimicrobial use, and proper design and use of instruments. Barrier isolation precautions aim to contain infection by reducing a hospitalized patient's physical contact with bacteria. Although such precautions are effective against bacteria from exogenous sources, they are of limited use in containing organisms endogenous to the patient. Furthermore, regardless of the quality of isolation precautions, they are ineffective in containing antimicrobial drug resistance which arises by mutation, genetic transfer, emergence, and selection of resistant strains. In addition, though manageable in a hospital setting, isolation precautions are often impractical in the community.
Avoiding the misuse of antimicrobials is also important in dealing with multiresistant organisms. While controlling the use of antimicrobials in hospitals may go a long way towards minimizing the problem, such control is difficult to enforce. Moreover, the benefits of controlled antimicrobial use in hospitals are often thwarted by the slower conformance in the community to judicious antimicrobial drug use.
An additional attack on problems of multiresistance includes proper design and use of instruments. Some organisms have attributes, independent of their ability to resist antimicrobials, which allow them to survive in or around instruments, thus making instruments a vehicle for dissemination of resistance and a reservoir of hospital organisms. Though better instrument design and use may reduce the dissemination of such organisms, it nevertheless does not address the development of antimicrobial drug resistance via mutation and selection.
What is needed is a new gene or gene product which can be targeted by classes of antimicrobials that are different from those currently used and to which microbial resistance is established. Discovery of such new genes and their products is particularly useful where they are present in more than one microbial strain, and found in both gram-negative and gram-positive microbes.